A detector, which detects such conditions of a wheel of a motor vehicle as the decrease in the air pressure of a tire on the wheel and the wear of the tire and then gives a warning, has been proposed. The detector, which rotates together with the wheel, needs a power supply such as a dry cell, a solar cell, or an electricity generator comprising a permanent magnet on the side of the chassis of the motor vehicle and a coil on the side of the wheel. A dry cell used as the power supply has a drawback that the life of the dry cell is so short that the dry cell needs to be replaced at regular intervals. The use of a solar cell as the power supply has a drawback that the solar cell is affected by the weather and time of the day so that the solar cell cannot produce any electricity in rainy weather and at night. The difficulty with using an electricity generator as the power supply secured to a wheel is that is is difficult in assembling the electricity generator to adjust the positional relation between the permanent magnet and coil at the electricity generator because the chassis and wheel of the motor vehicle are coupled to each other by an elastic suspension.